Composition for sic pack cementation coating of carbonaceous substrates

ABSTRACT

This invention relates to an improved pack mixture composition useful for the formation of a SiC pack cementation coating for protecting a carbonaceous substrate from degradation at temperatures above about 800° F. comprising: Si from about 15% to about 50% by weight of the total composition; B up to about 25% by weight of the total composition when present; SiO 2  from about 0.01% to about 3% by weight of the total composition; and SiC from about 40% to about 85% by weight of the total composition and method thereof. 
     The invention further relates to a cork release agent composition for providing the clean release of spent pack composition from a carbonaceous substrate.

FIELD OF THE INVENTION

The present invention relates to an improved pack mixture compositionused in the formation of a SiC pack cementation coating for impartingoxidation resistance to carbonaceous substrates.

BACKGROUND

Carbonaceous substrates, and more specifically carbon-carbon compositesare widely used in the aerospace and aviation industries due to thephysical and mechanical properties of such materials. The highstrength-to-weight ratio of carbon-carbon composites provides weightsavings to aircraft components. This characteristic, coupled with themechanical strength, toughness and dimensional stability ofcarbon-carbon composites to temperatures in excess of 4000° F. makesthese materials ideal for aerospace applications.

A potentially limiting characteristic of carbon-carbon composites isthat such materials tend to rapidly oxidize at temperatures above 800°F. in oxygen-containing environments. The degradation of the materialdue to oxidation causes loss of mechanical and physical properties. Thisproblem must be prevented in order for these materials to be effectivelyused for aerospace applications, such as turbine engine exhaust nozzlesand the space shuttle leading edge, where temperatures often exceed2000° F. for extended periods of time.

One solution to the problem of carbonaceous substrate degradation atelevated temperatures is to coat the substrate surface with SiC. Asilicon carbide coating effectively protects the carbonaceous substratefrom degradation by providing a barrier to oxygen diffusion. Coating thematerial with silicon carbide may be accomplished by pack cementation,chemical vapor deposition or other viable processing techniques.

U.S. Pat. Nos. 4,544,412 and 4,476,164 teach a powdered pack mixture anda pack cementation process used to produce SiC conversion coatings oncarbon-carbon materials. The pack mixture defined consists of roughly60% SiC powder, 30% Si powder, 1% B powder and 9% Al₂ O₃ powder.

U.S. Pat. No. 4,465,777 also discloses a boron-containing pack mixturecomposition for use in the protection of carbon-based materials. Thepack contains from 25% to 40% Si, from 50% to 70% SiC, and from 1% to15% B, with a minor amount of MgO, usually as an impurity in the boron.This reference further discloses the use of filter paper and filterpaper pulp release agents. Impregnation of the substrate withtetraethylorthosilicate and/or heat treatment prior to the cementationprocess in an attempt to increase coating pick-up is also disclosed.

U.S. Pat. No. 4,585,675 relates to pack cementation formulation ofsilicon carbide protective coatings for carboncarbon substrates. Thispatent teaches a powdered pack mixture composition of 40%-50% Si,30%-50% SiC and 20%-30% Al₂ O₃ that is reacted with the surface of acarbonaceous substrate to provide a primary SiC coating for protectionagainst the degradation of carbon-carbon substrates at hightemperatures. An enhancement coating, used to seal the primary coating,is also disclosed. This patent further discusses a hand tamped packingmethod and an alternative time saving process in which a slurry form ofthe reactive pack mixture is poured around the part in a graphiteretort.

Relatively recent technology improvements in the formulation anddensification of carbon-carbon substrates have made it increasinglydifficult to form a silicon carbide coating thereon with establishedpack cementation techniques. To enable coating of some of the newercarbon-carbon substrates, more reactivity is required of the packmixture.

Further, difficulty is typically encountered in removing the packmixture from the coated part surface after cementation particularly whenboron is a pack mixture ingredient. Attempts have been made by others tocircumvent the problem by keeping the boron content of the pack mixtureextremely low, i.e. less than about 1%, or by using carbonaceoussubstances such as filter paper or filter paper pulp as a pack releaseagent. The filter paper is deficient in that it shrinks and cracks awayfrom the part surface, directly exposing it to the reactive packingredients and resulting in pack sticking. Also, parts with complexconfigurations are difficult to wrap, making filter paper a pooralternative for use on such articles. Because of its fibrous nature,filter paper pulp tends to clog the spray devices used to apply it tothe part surface. A less expensive, more readily sprayable alternativematerial for a release agent is needed to address these problems.

Given the foregoing drawbacks, what is needed is a pack mixturecomposition that increases the degree of silicon carbide coatingformation. Further, a means of ensuring that such compositions willeasily release from the substrate after pack cementation is also needed.

It is, therefore, one object of the present invention to provide a packmixture composition for coating carbonaceous substrates which willimpede the degradation of the substrate by oxidation at hightemperatures.

It is another object of the present invention to provide a means for theclean release of expended pack mixture from the substrate.

These and other objects of the present invention will become apparent tothose skilled in the art in the description of the invention thatfollows, and in the examples and claims appended hereto.

SUMMARY OF THE INVENTION

This invention relates to a novel pack mixture composition comprising:Si from about 15% to about 50% by weight of the total composition; B upto about 25% by weight of the total composition when present; SiO₂ fromabout 0.01% to about 3% by weight of the total composition; and SiC fromabout 40% to about 85% by weight of the total composition, useful forthe formation of a SiC pack cementation coating for protecting acarbonaceous substrate from degradation at temperatures above about 800°F.

The invention also relates to a method for protecting a carbonaceoussubstrate from degradation at temperatures above about 800° F.comprising: preparing a pack mixture composition of from about 15% toabout 50% Si, up to about 25% B, from about 0.01% to about 3% SiO₂ andfrom about 40% to about 85% SiC, all by weight of the total pack mixturecomposition; coating the carbonaceous substrate with a release agent;contacting the release agent-coated carbonaceous substrate with the packmixture composition; and firing the carbonaceous substrate for a periodof time sufficient to effectuate the formation of a protective SiC packcementation coating on the carbonaceous substrate.

The invention further relates to a cork release agent composition forproviding the clean release of spent pack composition from acarbonaceous substrate.

DESCRIPTION OF THE INVENTION

In accordance with this invention, there is provided a novel packmixture composition useful for providing a SiC pack cementation coatingfor protection of carbonaceous substrates from degradation, includingand most prevalently oxidation. This pack mixture composition reactswith the substrate upon firing to convert a portion of the substratesurface into SiC which protects against the oxidation of the substrateat elevated temperatures, as high as 3000° F, and thus allows thesubstrate to maintain its mechanical integrity for longer periods oftime. The pack mixture composition comprises from about 15% to about 50%Si, from 0% up to about 25% B, from about 0.01% to about 3% SiO₂, theremainder of the composition comprising SiC, usually from about 40% toabout 85% SiC, all by weight of the total composition. The exactelemental composition may vary slightly depending on the substrate to becoated, and on the application for which and the conditions under whichthe article will be used, such as the temperature and the presence ofoxygen.

The term "carbonaceous" as used herein includes carbon-carbon compositeswhich are fiber reinforced carbon matrix materials, which may includeoxidation inhibitors in particulate or other form, as well as othercarbon and/or graphite substrates. The phrase "pack cementation" as usedherein refers to the heat driven conversion of outer surface carbon in acarbon-carbon composite to primarily silicon carbide by the infiltrationof and reaction with Si liquid and/or gas and SiO gas supplied by thepack mixture which surrounds the carboncarbon article.

The pack mixture composition disclosed herein is excellently suited foruse on carbonaceous substrates. The inclusion in or addition of SiO₂ inthe pack mixture enhances the conversion of substrate cafbon to SiC andresults in a thicker coating. It is theorized that the SiO₂ in the packmixture dissociates to form SiO and oxygen at pack cementationtemperatures. The SiO is free to react with the substrate carbon,thereby forming a protective layer of SiC and. evolving CO or CO₂ gases.The evolution of CO or CO₂ removes surface carbon and results in theexposure of additional substrate carbon available for reaction with SiOand Si vapor, thus facilitating additional SiC formation.

As was previously stated, the pack mixture taught herein contains about15% to about 50% Si, 0% up to about 25% B, about 0.01% to about 3% SiO₂and about 40% to about 85% SiC. Elemental Si can be purchased from Elkemas -200 mesh, the B, in amorphous form, may be purchased from Kerr McGeeas Trona™ at 1 micron, the SiO₂ may be purchased from Pennsylvania GlassSand Corp. as Supersil™ at -325 mesh, and the SiC (green) may bepurchased from Arendal as SIKA™ at 1200 grit. The preferred pack mixturecomposition contains from about 25% to about 40% Si, from about 0% toabout 15% B, from about 0.01% to about 1% SiO₂ and from about 44% toabout 75% SiC. The most preferred pack mixture composition containsabout 35% Si, about 5% B, about 0.5% SiO₂ and about 59.5% SiC. Thiscomposition is capable of converting both the carbon matrix and thecarbon fiber of the carbon-carbon composite at essentially equal rates,resulting in a dense coating with a relatively smooth interface.Variations in the pack mixture composition, especially SiO₂ content, cansignificantly affect the thickness of the conversion coating byinfluencing the reaction rate between the pack mixture and thesubstrate. For carbon-carbon substrates it is important that the amountof SiO₂ in the pack mixture composition generally not exceed 3% byweight of the total pack composition, as greater than that amount may betoo reactive, resulting in excessive conversion of the substrate.Preferably, the amount of SiO₂ in the pack mixture does not exceed 1% byweight of the total pack composition.

In some instances, it is desirable to completely convert a carbonaceoussubstrate to silicon carbide. This can be achieved with somecarbonaceous substrates, especially those which are highly porous, byconversion with a SiO₂ -containing pack mixture composition.

Prior to application of the pack mixture composition to the carbonaceoussubstrate, it is usually preferable to coat or cover the substrate witha pack release agent. The release agent aids in the removal of any packmixture material adhering to the part surface after the cementationprocess has been completed. The need for a release agent is particularlyimportant when using boron containing pack mixtures which tend to adhereto the substrate. Further, choice of an appropriate release agent tendsto promote the vapor phase reaction, thus enhancing the conversionreaction.

The preferred release agent according to the subject invention is cork.The material is typically purchased with a -200 mesh particle size and adensity of between 8 to 10 lbs./ft³, and is commercially available fromthe Maryland Cork Co., Inc. However, a variety of particle sizes anddensities will be effective for the purpose stated herein. For ease ofapplication, the powdered cork is mixed with a liquid carrier, such as0.4% aqueous solution xanthan gum. The xanthan gum can be purchased asKelzan-S™ powder from Kelco Co.

Cork allows the reaction of the pack mixture composition with thesubstrate to proceed, then provides for the clean release of the packmixture remaining after completion of the pack cementation process,without damaging the conversion coating, and consequently not exposingthe substrate material. The cork prevents pack sticking by converting toa very porous low strength silicon carbide layer which is friable andtherefore easily removable from the substrate surface.

Pack release agents, in general, tend to degrade and shrink duringcementation forming shrinkage cracks. It is critical that the typicallyformed shrinkage cracks are not large enough to permit direct exposureof the carbonaceous substrate to the pack mixture. This is especiallytrue of filter paper or other sheet-like release agents, which tend totear, resulting in opposing edges of the release agent pulling away fromeach other upon continued shrinking. This increases the chance that thesubstrate surface will be exposed and bond to the pack mixture. Cork,however, due to its particulate nature, undergoes only minor shrinkage,which results only in a very fine crack structure. This prevents directexposure of the substrate to the pack mixture, effectively preventingthe pack mixture from bonding to the surface of the substrate.

Other practical benefits of using cork as a release agent include itsrelatively low cost (less than one-tenth that of filter paper products)and the ease with which the powdered cork can be mixed with abinder-containing aqueous or non-aqueous liquid carrier, such as 0.4%aqueous solution xanthan gum, and sprayed, painted or dipped onto asubstrate to any desired thickness. Further, the porous film of asprayed on cork release agent does not impede the reaction between thecarbonaceous part and the pack mixture. Finally, cork is highlyadvantageous when the material being coated is of a complexconfiguration since it will conform to any surface. Filter paper orother pack release agents may be impractical in such instances.

Once the release agent has been applied to the substrate, the packmixture may be applied over the release agent by any known method,including hand tamping of dry pack mixture, slurry fill technique, orspray coating, among others. When using the slurry fill technique or thespray coating technique it is desirable to combine the pack mixture witha liquid carrier, so that the mixture is in a fluid form. Such liquidcarriers may be aqueous or non-aqueous solvents, and may further containsuitable binder materials or suitable dispersants.

When using the dry pack mixture technique, the release agent-coatedsubstrate is loaded into a non-reactive retort that is filled with packmixture such that the mixture covers all sides of the substrate to athickness of at least about 1/2 inch. The pack mixture is hand tamped toattain a density which ensures that the substrate is fixed in place andis uniformly surrounded by enough reactive pack mixture to attain thedesired conversion coating, preferrably at least about 1/2" thick, mostpreferrably about 3/4" thick. The density is typically measured by usinga soil tester.

When using the slurry fill technique, the release agent-coated substrateis loaded into a non-reactive retort. The retort may contain a smallamount of slurried pack mixture prior to placement of the substrate inthe retort, or alternatively, the slurried pack mixture may all be addedafter placement of the substrate in the retort. Either way, the resultachieved is that the substrate is surrounded by a uniform amount ofslurried pack mixture on all sides. The retort containing the substrateis then dried in air or in an oven for a time sufficient to dry theparticular pack mixture used.

When using the spray coating technique, the release agent-coatedsubstrate is first sprayed with a thin coating of pack mixture. Thesubstrate is then dried. This process may be repeated several times inorder to attain a pack mixture coating of a desired thickness. Thesubstrate may then be placed in a non-reactive retort filled with acourse grade SiC powder such that the substrate is surrounded by thepowder. Alternatively, the pack-coated substrate may be placed directlyinto a furnace without first encasing it in a retort. The spray coatingtechnique readily lends itself to masking of the substrate in selectedareas to prevent reaction with the pack mixture when such is desired.

In each of the above methods, the packed retort, or the pack-coatedsubstrate without the retort, is placed in a furnace which is heated toa temperature ranging between about 2900° F. and about 3300° F. Thistemperature is then held for a period of about 4-16 hours, depending onthe reactivity of the substrate and the amount of coating pick-updesired.

Firing of the substrate should take place in an inert atmosphere,preferably argon, at slightly above atmospheric pressure.

The furnace is then cooled in an inert atmosphere until it reaches roomtemperature, at which time the retort or pack-coated substrate can beremoved. If a retort was used, the retort is opened and the substrateremoved therefrom.

The conversion coated substrate is then removed from the spent packmixture, a process made easy by prior use of the novel cork releaseagent, and cleaned by rubbing the surface with an abrasive pad andrinsing it with an appropriate solvent, such as isopropyl alcohol.

The substrate now has a conversion coating, of between about 0.1-50 milsdepending on which of the listed cementation techniques was used, theroughness of which is similar to the original substrate surface. TheSiO₂ -bearing pack mixture composition of the current inventiongenerally results in increased coating thickness as compared to non-SiO₂-bearing compositions.

EXAMPLES

The following examples demonstrate the formation of a SiC packcementation coating on a carbon-carbon substrate using the novel packmixture composition disclosed herein. It is to be understood that theseexamples are utilized for purposes of illustration only, and are notintended, in any way, to be limitative of the present invention.

For purposes of these examples, the substrates used were a series ofcompositionally identical carbon-carbon composite coupons. Each wascleaned to ensure that no foreign matter was present to interfere withthe cementation process. This was accomplished by first scrubbing thecoupon, then wiping it down with isopropyl alcohol and finally ovendrying the coupon for 15-30 minutes at about 190° F. Pack cementationprocessing, as described in Examples 1-4, was then used to coat thecoupons.

EXAMPLE 1

A cleaned coupon was spray coated with a cork release agent according tothe following procedure. The cork release agent, which consisted of an0.4% aqueous solution xanthan gum and powdered cork in a ratio of about7.5:1, was prepared by vigorously mixing the cork solution in a paintmixer for 15 minutes. This solution was sprayed over the entire surfaceof the coupon with an air brush. The release agent coating was thendried in an air oven at 190° F. for about 15 to 30 minutes. The coatingwas approximately 5-10 mils thick. A second coating of cork wassimilarly applied, increasing the total thickness of the cork to about15 mils, to ensure that the entire substrate surface was covered.

The pack mixture was prepared by blending the following powders in atwin-shell mixer for 30 minutes: 59.5% SiC green (1200 grit), 5%amorphous B (about 1 micron), 35% Si (-200 mesh), and 0.5% SiO₂ (-325mesh). A slurry was prepared by combining the pack formulation with 0.4%aqueous solution xanthan gum in a ratio of 1:1 by weight, and mixing ina paint shaker for 15 minutes. The slurry was then sprayed over thedried cork release layer to a thickness of about 50 mils. Several sprayapplications were necessary to achieve this thickness.

The coupon was then dried at about 190° F. in an air oven for about 30to 45 minutes. It was then loosely packed in 240 grit SiC powder in agraphite retort. The retort was fired in a resistance heated furnace for8 hours at 3200° F. in an argon atmosphere at slightly above atmosphericpressure.

After the cementation process was completed, the 240 grit SiC powder wasremoved from around the slurry encased coupon. The slurry had formed ahard shell around the coupon during firing. The coupon was easilyremoved by merely cracking off the slurry shell. Any cork release agentresidue was easily brushed from the coupon surface. The coupon wasrubbed lightly with an abrasive pad and then cleaned with isopropylalcohol. The coated coupon surface was smooth and did not appearroughened by the conversion process. A cross-section of the couponrevealed a SiC conversion coating had formed with a uniform thickness ofabout 0.25 mils.

EXAMPLE 2

A coupon prepared identically to the coupon in Example 1, up to andincluding application of the cork release agent, was used for thisExample 2. The pack mixture composition for this Example was identicalto that used in Example 1, however for this Example the pack mixture wasused in a dry state.

The coupon was then packed in a cardboard-lined graphite retort suchthat there was 3/4" pack mixture surrounding the coupon on all sides andsurfaces. The pack mixture was hand tamped to achieve a pack mix densityof approximately 1.32g/cc. The packed retort was heated to 3200° F. andsoaked for 8 hours in an Argon atmosphere at slightly above atmosphericpressure.

After the cementation process was completed, the retort was disassembledand the fired pack mixture was fractured. The coupon was easily removedand cleaned as in Example 1.

EXAMPLE 3 AND 4

The processing used in Example 2 was repeated on identical couponsubstrates using previously patented pack mixtures containing 60% SiC(green), 35% Si and 5% B for Example 3, and 60% SiC, 30% Si, 1%B and 9%Al₂ O₃, for Example 4.

Table 1 below shows a comparison of Examples 2, 3 and 4, all of whichwere processed using a dry pack mixture. As can be seen, the novel SiO₂containign pack mixture of Example 2 produced the greatest weight gainper unit area of substrate surface. Thus, the inclusion of SiO₂ in thepac mixture resulted in a thicker coating.

                  TABLE 1                                                         ______________________________________                                                                              Mass Change                                                                   per Unit                                     Pack Mix  Surface  Initial                                                                              Final  Surface                                      Com-      Area     Mass   Mass   Area                                    Ex.  position  (cm.sup.2)                                                                             (grams)                                                                              (grams)                                                                              (mg/cm.sup.2)                           ______________________________________                                        2    59.5% SiC 62.053   28.4387                                                                              32.4848                                                                              65.204                                       35% Si                                                                        5% B                                                                          0.5% SiO.sub.2                                                           3*   60% SiC   48.650   13.4619                                                                              14.4205                                                                              19.704                                       35% Si                                                                        5% B                                                                     4**  60% SiC   61.893   28.3735                                                                              28.9988                                                                              10.103                                       30% Si                                                                        1% B                                                                          9% Al.sub.2 O.sub.3                                                      ______________________________________                                         *composition from patent #4,465,777.                                          **composition from patent #4,544,412.                                    

It is to be understood that the forgoing examples have been provided toenable those skilled in the art to have representative examples by whichto evaluate the invention and that these examples should not beconstrued as any limitation on the scope of this invention. Inasmuch asthe composition of the pack and the processing parameters employed inthe present invention can be varied within the scope of the totalspecification disclosure, neither particular components, relativeamounts of the components, nor exact operating parameters exemplifiedherein shall be construed as limitations of the invention.

What we claim is:
 1. A method for protecting a carbonaceous substratefrom degradation at temperatures above about 800° F. comprising:a)preparing a pack mixture composition of from about 15% to about 50% Si,0% up to about 25% B, from about 0.01% to about 3% SiO₂ and from about40% to about 85% SiC, all by total weight of said pack mixturecomposition; b) coating said carbonaceous substrate with a releaseagent; c) surrounding said release agent-coated carbonaceous substratewith said pack mixture composition; and d) firing said carbonaceoussubstrate for a period of time sufficient to effectuate the formation ofa protective SiC pack cementation coating on said carbonaceoussubstrate.
 2. The method as in claim 1 wherein said compositioncomprises about 25% to about 40% Si, 0% up to about 15% B, about 0.0.1%to about 1% SiO₂ and about 44% to about 75% SiC, all by total weight ofsaid coating composition.
 3. The method as in claim 1, wherein saidcomposition comprises about 35% Si, about 5% B, about 0.5% SiO₂ andabout 59.5% SiC, all by total weight of said coating composition.
 4. Themethod as in claim 1 wherein said pack mixture composition is in apowder form.
 5. The method as in claim 4 wherein said powder compositionis applied to said substrate by a dry pack mixture technique.
 6. Themethod as in claim 1 wherein said pack mixture composition is combinedwith a liquid carrier and is in fluid form.
 7. The method as in claim 6wherein said fluid composition is applied to said substrate by a slurryfill technique.
 8. The method as in claim 6 wherein said fluidcomposition is applied to said substrate by a spray coating technique.9. The method as in claim 1 wherein said release agent is a slurrycomprising cork suspended in a suitable binder-containing liquidcarrier.
 10. The method as in claim 9 wherein said binder-containingliquid carrier is 0.4% aqueous solution xanthan gum.
 11. The method asin claim 1 wherein said firing is at about 2900° F. to about 3300° F.12. The method as in claim 11 wherein said firing is at about 3200° F.13. The method as in claim 1 wherein said period of time sufficient toeffectuate the formation of a protective coating on said carbonaceoussubstrate is about 4 to about 16 hours.
 14. The method as in claim 13wherein said period of time is about 8 hours.